Method of making heat exchange apparatus



Nov. 20, 1945. J.' E'. wooDs METHOD 0F MAKING' HEAT EXCHANGE APPARATUS Filed Oct. 7, 1942 Patented Nov. 20, 1945 METHOD or MAKING HEAT EXCHANGE APPARATUS John E. Woods, Brookline, Mass., assignor to Clifford Manufacturing Co., Boston, Mass., a corporation of Delaware Application October 7, 1942, Serial No. 461,109

4 Claims.

made by assembling hexed end tubes (usually of copper) in a honeycomb bundle and soldering the hexed ends together, usually by dipping in soft solder. The tanks or casings are attached to the outside tubes of the bundle, also by soft solder.

By soft solder is meant any of the commercially available solders having melting points approxichange apparatus is constructed of thin-wall `closely spaced tubes by assembling the tubes in `to copper, aluminum, and other non-ferrous mately in the range of 381 to 595 F. The appaoutside rows, and result in the appearance of leaks. These mechanical stresses are frequently due to vibration effects, since apparatus of this kind finds its most important use in the aircraft industry.

One of the principal reasons for the use of soft solder is to prevent destruction of the tubes in the dipping operation. Hard or high melting solders or brazing compounds, with melting points ranging from 11'75" to 1600 F., are commercially available, but the manufacture of heat exchangers from thin-wall tubes by the use of such solders is not feasible because the dipping operation in a bath of such material would result in immediate destruction of the thin tube ends by dissolution in the large mass of brazing compound.

The difculties noted above are even greater in the case of aluminum, which would be highly desirable because of its lightweight in the manufacture of equipment for use in aircraft. The only available brazing alloys for use with aluminum have melting points of the order of only 100 F. lower than that of the aluminum itself, and any standard soldering or brazing operation by dipping or otherwise can not be carried out without immediate destruction of the tubes. Y As a consequence, no successful manufacture of heat exchangers with thin-wall tubes has heretofore been carried out.

According to the present invention, heat ex-` metals or alloys, as desired, and has been found to avoid most of the disadvantages inherent in the earlier construction.

The tubes are straight; and of uniform diameter, and are therefore relatively inexpensive, not only because of the small number of manufacturing operations required, but also because they are not subjected to damage by initial hexing or bulging operations. The mechanical stresses applied to the tanks are taken up by the heavy headers and are not directly communicated to the tube assembly, as will appear in the specific description.

Although header constructions are not new, their use vhas been mainly limited to boilers,O economizers, etc., employing thick tubes which may be expanded into the headers. Such expanding operations are not feasible with thin- Wall tubes, with which thel present invention is concerned. By thin-wall tubes is meant tubes which are usually of the order of .005" and in no case greater than 14,4" in wall thickness.

Header constructions have also been proposed for widely spaced tubes. in which the tubes may be securely supported in drifted or flanged holes. This construction is likewise not feasible for closely spaced tubeshwherein the distance be- .tween tube centers in any row is only slightly greater than the tube diameter. So far as I am aware, no header construction with closely spaced thin-wall tubes has heretofore beensuccessfully made. f

Preferably the tubes are secured to the headers by brazing or hard soldering, and one feature of the invention contemplates the use of a brazing or soldering compound in measured amounts,

suicient to establish an adequate bond, but with- The invention is applicable to aluminum v In the accompanying drawing, Fig. 1 is'a perspective view of a core assembly according to the present invention; Fig. 2 is a detail view illustrating the method of preparing the assembly for the brazing operation; and Fig. 3 is a sectional view of the complete assembly.

In the illustrated embodiment of the invention, the tubes I0, which are drawn or extruded from copper, brass, aluminum, aluminum alloy .or other non-ferrous metal, are inserted in closely spaced punched holes in the headers l2 and I4. The tubes are 'of thin-wall structure on the order of .005" to .007" in wall thickness. The headers are either of the same material as the tubes or of material compatibletherewith. For example, brass, copper or Monel headers may be used with copper tubes, and aluminum or aluminum alloy headers with aluminum tubes. The headers comprise flat sheets provided with peripheral :flanges I6 to stien the sheets and provide for attachment to the shell or tank. The tube receiving holes in the headers are closely spaced, the spacing between holes in any row being only slightly greater than the tube diameter. As an example, one construction in commercial use employs tubes .268" in diameter spaced .313" on centers.

An intermediate header I8 may be used for affording additional support for long tubes, for more secure attachment to the tank, for bailing, or other purposes. In apparatus of the conventional headerless type, the intermediate support has been provided by forming small bulges in the intermediate portion of the individual tubes so that the bulges of adjacent tubes contact one another, but -that method is unduly expensive. In the present invention, one or more intermediate supporting plates may easily be included in the assembly. The headers and intermediate plates are supported the proper distance apart in a, suitable jig, with the holes in register, and the tubes are passed through the holes. The end portions of the -tubes protrude uniformly, as shown in Figs. 2 and 3.

For the bonding of the tubes to the headers, a measured amount of high melting brazing compound is provided at the joints.` The brazing compound is only suilicient in amount to afford the proper alloying action and penetration of the compound into the material of the tubes and headers, but without danger of tube destruction.

The preferred method of applying the brazing material to the assembled tubes and headers consists in using brazing material in the form of loose granules or gravel, as shown in Fig. 2. For a copper assembly, the granules are in the form of silver alloy or silver solder having a melting point of 11'75 F., or perhaps somewhat higher. For an aluminum assembly, granules of aluminum brazing alloy are preferably used. The

assembly of headers and tubes is then set up with the headers in a substantially horizontal position, after which the granules of brazing, mateuniformly distributed over the particles of brazing' alloy and around the tubes.

' The assembly is then placed in a brazing oven -with the tubes vertical and is brought to a temperature 'suiicient to melt the brazing compound. 'I'he upper header and the intermediate plate are supported by a few beaded tubes 26, inserted at the start of the tube-assembling operation. In the brazing oven the tubes are heated by circulation of hot air therethrough. Since the braz' ing compound is on the upper side of the top header, proper flow of compoundinto and around the tube joints by gravity is assured.

' Two operations are required. After one end of the assembly is treated as above described, the

assembly is taken out and turned over, and the other end is provided with alloy and ilux and the assembly is heated for the second brazing operation. Since the oven is heated by down currents of hot air or gas, the top of the assembly is heated more rapidly than the bottom, and consequently the brazing may be completed at one end without melting the compound at the other end. This method is advantageous for aluminum equipment because it doesnot require prolonged overheating of the assembly.

The method therefore provides a heavy header construction in which al1 of the joints are formed by a hard solder or brazing alloy, as distinguished i hexed tubes.

from the soft solder method commonly used on In the present invention, the amount of brazing compound is only sufficient to insure penetration thereof into the tube walls to an extent to form proper bonds, but without danger of tube destruction.

Since each tube is surrounded by brazing material as a result of the uniform distribution of the granules, the heat conduction to all parts of the tubes and headers is excellent. In addition, since the granules of brazing material and of flux are in intimate association throughout the mass, the brazing material readily bonds to all portions of the joints as soon as the temperature of the parts has reached the required temperature.

For aluminum, the granules of brazlng alloy and of ux are of approximately the same density. with the result that the flux and alloy become mixed upon heating, and the flux is available at the bonding surface. The particles of brazing compound and flux originally nll up most of the spaces between the tubes, since they are in loose form, but upon melting, the brazing compound compacts into a thin layer which bonds tightly with the headers and tubes.

The method of Flg. 2 has been found to possess some decided advantages. It has been noted that one of the difiiculties in manufacture has been the tendency for the brazlng compound to flow toward certain joints and away from others. This robbing" action seems to be due in some measure to non-uniform heating of the parts. Actually this is aserious dimculty, since the tubes, being relatively thin, heat more rapidly than the headers and tend to draw the brazing alloy away tool, which forms the ends of a number of the tubes simultaneously. Although some of the tube ends maybe split by this operation, the splits are or no consequence, since the only purpose oi the hexing operation is to enlarge the face area, no reliance being placed on the hexed ends ior uid seals or structural rigidity. Thu's the aerodynamic advantages of the common honeycomb radiator are obtained, but with the additional advantages arising from the header construction.

The t or shell d@ (Fig. 3), having the usual :duid inlet and outlet connections l2, is preferably applied as the :dnal step. 'I'he tank is placed over the tube and header assembly and is securely attached to the header iianges I6, conveniently by torch brazing. Since the tank is attached-to the headers, instead of to the relatively fragile tubes, it is unnecessary to rely on soft solder. Owing to the thickness of the header and tank material, the connection may be made with hard solder or brazing compound, without damage. If desired, the attaching operation may be carried out by iine welding, either instead of or in addition .to the brazing process.

The steps above described need not be carried out in the exact order specified. For example, the hexing of the tube ends may beperformed after the attachment of the tank.

Heat exchange apparatus made according to the present invention is characterized by great structural strength and rigidity. Mechanical stresses applied to the structure, by vibration or otherwise, are borne by the heavy tank and headers and are not communicated to the tubes or to the joints betweenthe tubes and headers. 'I'he bonds between the tubes and headers, and between the headers and tank, have high mechanical strength and are of sumciently high melting pint to permit use of the equipment under the most severe temperature conditions. Furthermore, such equipment is not limited to copper constructions, but may be used with other materials, notably aluminum, which is advantageous because of its light weight.

The intermediate plate i8, which has been described as a tube-supporting` plate, may also be used as a baille or bulkhead. One or more of such plates may be employed for baiiling purposes, and may be of such contours as to direct the ow of fluid in any desired manner externally of the tubes. It will be understood that bailles or intermediate plates cannot be'used in the conventional construction employing tubes whichare necessarily expanded at their ends before assembly. In the present case, where tubes of uniform cross- 3 section are used (except for a relatively'small number of tubes with single bulges for supporting purposes), the assembly with baiiles and intermediate plates of any desired number and coniiguratlon presents no dimculty.

Having described my invention, I claim:

1. In the manufacture of heat exchange apparatus comprising a plurality of closely spaced thin-wall metal tubes mounted in headers having `unanged tube receiving openings, the method of bonding the tubes to the headers which comprises applying brazing material to the headers in the form of loose discrete particles of the general size of 0.010'! to 0.025", distributing the brazing material aroundthe tubes to a substantially uniform depth on the headers, applying flux above the brazing material in the form of particles of small size, and thereafter raising the assembly to brazing temperature.

2. In the manufacture of heat exchange apparatus comprising a plurality of closely spaced thin-wall metal tubes mounted in headers having unanged tube receiving openings, the method of bonding the tubes to the headers which comprises applying brazing material to the headers in the form of granules. applying iiux in granular form above the brazing material, distributing the granules of brazing material and iiux uniformly over the header and around the tubes, and thereafterA raising the assembly to brazing temperature.

3. In the manufacture of heat exchange apparatus comprising a plurality of closely spaced thin-wall tubes of light metal mounted in headers having unanged tube receiving openings. the method of bonding the tubes to the headers which comprises applying brazing material in granular form to the headers and between the tubes, applying flux in granular form above the brazing material, the ux and brazing material being of approximately similar densities; and thereafter raising the assembly to brazing temperature.

4. In the manufacture of heat exchange apparatus comprising a plurality of closely*l spaced thin-wall metal tubes mounted in headers having unanged tube receiving-openings, the method of bonding the tubes to the headers which comprises applying brazing material to the headers in the form of granules, applying flux above the -brazing material in the form of particles of small size, distributing the granules of brazlng material and flux uniformly over the header and around ythe tubes, and thereafter raising the assembly to brazing temperature.

JOHN E. WOODS. 

